email_notifications: Rename append_sender_to_message to prepend.
email_notifications: Rename append_sender_to_message to prepend.

The sender goes on the beginning, which is prepending to the message.

zulip/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,zulip/zulip,kou/zulip,rht/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,zulip/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,kou/zulip,zulip/zulip,kou/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip,eeshangarg/zulip